EQNEWS / December, 1996
Cover Photograph:
Wrightwood, California, M+7 Earthquake, December 8, 1812

Cover Story: On December 8, 1812, a powerful earthquake registering +7.0 occurred on the Mojave segment of the San Andreas fault. The exact location of the epicenter is not known, however, surface ruptures are occurred in Wrightwood, located along the northern flank of the San Gabriel Mountains, about 70 miles northeast of Los Angeles. Geologic investigations and historic accounts of the earthquake suggest surface ruptures may have extended at least 16 miles northwest of Wrightwood and possibly as far south as San Bernardino Valley. Significant damage was reported as far away as San Juan Capistrano, where the foor of the mission collapsed and 40 persons were killed.

An exposure at Swarthout Creek, located about 3 km northwest of Wrightwood, shows the San Andreas fault cutting through interbedded layers of peat (marshy plant remains) and silts, sands, and gravels (See photograph). This exposure, along with other backhoe exposures, were described by geologists T. E. Fumal, S. K. Pezzo-pane, R. J. Weldon II, and D. P. Schwartz of the U.S Geologic Survey in a report published in Science, Vol. 259, 8 January 1993, "A 100-Year Average Reccurrence Interval for the San Andreas Fault at Wrightwood, California". They inferred that the peat, silts, sands, and gravels sequences were deposited in a structural pull-apart basin that had been accumulating the marshy sediments for about 5,000 years before being drained by the incision of Swarthout Creek within the last century. They discovered fault ruptures had penetrated to specific layers of the sediments suggesting the faulting had to have occurred prior to the deposition of the overlying unfaulted sediments.

Radiocarbon dating of the peat-layers revealed the ages of the sediments and therefore the approximate ages of the earthquakes that caused the fault ruptures. The geologist's data showed at least 5 very strong (M+7?) earthquakes had during the past 500 years with the approximate ages of: 1857, 1812, 1690, 1610, and 1470. The key to sequence are the 1857 and 1812 earthquakes, the last major earthquakes along the southern San Andreas fault during historic time! The data suggests average time between very strong events is 97 years, with the greatest being 140 years and the shortest in just 45 years. Disturbingly, it has been 140 years since this segment last ruptured!

Image: Tom E Fumal, U.S. Geological Survey.

Rev. Jan. 12, 1997